When you're using the mic, try and keep it behind the screen rather than in the wind - it does make a difference. Also, if it's raining or you're taking lots of spray, blow out the mic first to clear any water that may be garbling comms. When you get back, rinse the thing out as well to prevent the salt from drying and also garbling comms (we found this with the safety boats at the yacht club).
My Icom 503 has been (and still is - touch wood) faultless, and I can hear it clearly at any speed since it's right down in front of me.
A personal gripe is the stupid little rib aerials with the coils(!!) - after having worked on ribs with these, and then going back to my own boat with 1.8 metre fibre glass antennae I then realise how much better performance I get with both reception and transmission due to the extra height.
Before anyone says, my aerials are now over 4 years old, and they get subject to pretty extreme movements at the back of the boat since we only ever seem to go out when the swell is over 3metres high